The present invent ion relates to a ladder-type electric filter device including a plurality of piezoelectric resonators arranged in series and parallel and suited for use with portable and mobile radio communication equipments.
A ladder-type electric filter device of the above described type normally comprises a plurality of L-connection circuits as basic unit circuits arranged in layers, each comprising a thick and small capacity series-resonator and a thin and large capacity parallel-resonator that are L connected and contained in a casing. Such an arrangement is disclosed in U.S. Pat. No. 4,864,259.
FIGS. 1 to 4 of the accompanying drawings illustrate a typical electric filter unit as disclosed in U.S. Pat. No. 5,057,802, its basic unit circuit comprising four resonators.
A shallow and substantially cuboidal casing 1 made of synthetic resin has an opening 2 at a front side portion and an inner space 3 for containing components of the filter device. The casing 1 is formed integrally with a grounding terminal plate 4 by insert molding, the terminal plate 4 having a connector leg 4a projecting outwardly and rearwardly from the rear wall portion of the casing 1.
The components of the filter circuits to be housed in the casing 1 are inserted into the inner space 3 of the casing 1 through the opening 2 and disposed above and below the grounging terminal plate 4. More specifically, a first buffer plate 5, an input terminal plate 6 provided at a front edge with a connector leg 6a, a first series-resonator 7, a first interconnecting terminal plate 8, a first auxilially terminal plate 9 and a parallel-resonator 10 are successively disposed above the grounding terminal plate 4 in this order from above.
Then, a second interconnecting terminal plate 11, an insulating plate 12, a third interconnecting terminal plate 13, a second series-resonator 14, an output terminal plate 15 provided at a front edge with a connector leg 15a, a second auxiliary terminal plate 16, a second parallel-resonator 17, a fourth interconnecting terminal plate 18 and a second buffer plate 19 are successively disposed below the grounding terminal plate 4.
The first and third interconnecting terminal plates 8 and 13 are connected with each other by a connecting strip 20 which is arranged at a front corner thereof and integrally formed. Similarly, the second interconnecting terminal plate 11 is connected to the fourth interconnecting terminal plate 18 by a connecting strip 21 which is arranged at a front corner thereof and integrally formed. The connecting strips 20 and 21 are positioned within lateral indentations 3a and 3b which are respectively provided on the lateral wall portions of the casing 1 near the opening 2 to partly define the inner space 3 and prevent the connecting strips 20 and 21 from contacting with any of the other components. When the filter components are housed in the casing 1, the connector leg 6a of the input terminal plate 6 and the connector leg 15a of the output terminal plate 15 project outwardly and forwardly from the respective opposite front corners. Then, sealing or cover plate 22 is fitted to the opening 2 of the casing 1 with the outwardly extending connector legs 6a and 15a running through respective through holes 22a and 22b, and the casing 1 is hermetically sealed by sealing agent or filler 23 such as epoxy resin applied to the sealing plate 22 to finish the operation of assembling a ladder-type electric filter unit.
As shown in FIG. 3, the filter unit having a configuration as described is then mounted on a print-circuit board 24 with the portions of the connector legs 6a, 15a and 4a folded in grooves 1a, 1b and 1c on the lower surface of the synthetic resin casing 1 being bonded to respective conductors of the print-circuit board 24 by means of soldering generally designated by the reference numeral 25.
The filter unit as described above essentially comprises a pair of basic filter unit circuits arranged in two layers, each including one series-resonator and one parallel-resonator which are L connected. Thus, a multi-layer filter circuit can be obtained by cascade connecting a plurality of such filter unit. FIG. 4 shows an equivalent circuit diagram of the ladder-type electric filter arrangement comprising tow filter units.
While ladder-type electric filter devices of the above described kind are popularly used for filter circuits of radio communication equipments of various types, they are required to show improved group delay characteristics along with excellent amplitude characteristics that they already have if they are used for digital modulation type mobile radio stations such as automobile and portable telephone sets.
The requirement for the improved group delay characteristics has been met, through not very satisfactorily, through improvement of the group delay characteristics of the ladder-type electric filters by using resonators of piezoelectric ceramic material having a small mechanical quality factor Qm. However, the resonators with such a small mechanical quality factor Qm are inevitably accompanied by a disadvantage of remarkably increased insertion loss as compared with resonators with a large mechanical quality factor Qm.